The collection of subcutaneous ECG signals is to be distinguished from collection of electrogram (EGM) signals. The latter requires the use of intracardiac leads implanted in the myocardium, for the measurement of a depolarization potential, mainly for the purpose of controlling a cardiac stimulator. As for the ECG signals, they are intended to be recorded over a relatively long period of time, so as to be further processed and analyzed using reconstruction algorithms to assess the patient's clinical status, and eventually diagnose a heart rhythm disorder.
In the case of collecting subcutaneous ECG signals, the collected data are accumulated in a memory of the implanted device. Then, so as to allow them to be analyzed, the memorized data are transferred from the implanted device to an external device, or “programmer”, that functions to verify the parameterization of the implanted device, read the recorded information or write data, and/or update the internal software of the implanted device.
That data exchange can be performed through telemetry, that is: through a technique of remote data transmission with no galvanic contact.
Up to now, telemetry used to be essentially performed through magnetic coupling between a coil of the implanted device and a coil of the programmer (or programming head), a technique known as an “induction technique”. Recently, another non-galvanic coupling technique has been proposed, using the two components of an electromagnetic wave produced by transmitter/receiver circuits operating within the radiofrequency (RF) domain, typically with frequencies ranging around several hundreds of Megahertz. This technique, known as “RF telemetry”, allows one to program or interrogate implanted devices from distances greater than 3 m, and therefore allows the exchange of data without the need for using a telemetry head, and indeed even without the intervention of an external operator.
U.S. Pat. No. 5,331,966 discloses an implanted device comprising electrodes for collecting subcutaneous ECG signals placed on its case, as well as means transmitting collected data using RF frequency towards an external device. The electrodes are positioned partly on the connector head, and partly on the case as such. Indeed, for a sufficiently efficient collection of the signals, the different electrodes shall be placed apart, as far away from each other and in as much of an orthogonal configuration as possible. U.S. Pat. No. 5,331,966 also refers to placing the electrodes on the edge of the case.
The latter arrangement of electrodes is also referred to in U.S. Pat. No. 6,522,915, which refers to a mounting element in the shape of a surround shroud placed around the periphery of the case and having over its external surface, electrodes in the shape of spiral wires. The implanted device includes an RF telemetry antenna, either within the case or placed on the connector head.
The devices described in these prior art documents present a relatively complex and very specific structure, notably so as to take into account the electrodes placement, which is an essential point for a satisfactory collection of ECG signals.
When it comes to equip an existing model of an implantable device with features for subcutaneous ECG signal collection, these constraints impose a redesign of the case so as to embed the electrodes (as in U.S. Pat. No. 5,331,966 referred to above) or use a follower element dedicated to the collection of ECG signals (as in U.S. Pat. No. 6,522,915 referred to above).
Furthermore, if it is wished to implement an RF telemetry system, the arrangement of the antenna relative to the case is also critical in order to obtain a satisfactory radiation pattern, respecting the conductive character of the case's body. By comparison, the induction telemetry technique is a more robust technique which is far less sensitive to such issues.
This is the reason why RF antennas of implanted devices are commonly placed in the connector head, because an antenna enclosed in the metal case could not radiate in a satisfactory manner.